1. Field of the Invention
The invention relates to a zoom lens, and more particularly, to a zoom lens suitable for a photographic lens used in an image pickup apparatus such as a digital still camera, a video camera, a monitoring camera, a broadcasting camera, or a silver-halide film camera.
2. Description of the Related Art
In recent years, image pickup apparatuses such as digital still cameras, video cameras, monitoring cameras using a solid-state image sensor, or silver-halide film cameras have been miniaturized while retaining high functionality. In a photographic optical system used in an image pickup apparatus of the type listed above, there is a demand for a zoom lens that is compact, has a short lens length, and high resolution throughout the zooming range. In addition, in order to obtain a wide image-sensing range even in a short shooting distance, the zoom lens is required to have a wide viewing angle and a short focal length at the wide-angle end.
There is known in the current state of the art, a four-unit zoom lens including a first lens unit of a positive refractive power, a second lens unit of a negative refractive power, a third lens unit of a positive refractive power, and a fourth lens unit of a positive refractive power in order from an object side to an image side. U.S. Pat. No. 7,760,441 and U.S. Pat. No. 7,830,613 discuss a rear-focus type four-unit zoom lens in which zooming is performed by moving each lens unit, and focusing is performed while an image plane variation caused by the zooming is corrected by moving the fourth lens unit.
In order to obtain high optical performance across the entire zoom range along with a wide viewing angle and a high zoom ratio in the zoom lens used in the image pickup apparatus, it is important to appropriately set a zoom type, refractive powers of each lens unit, lens configurations of each lens unit, and other like parameters. For the four-unit zoom lens described above, it is important to appropriately set a lens configuration of each lens unit, movement amounts of the first and third lens units caused by the zooming, refractive powers (inverse of the focal length) of the first and third lens units, and the like.
In addition, it is important to appropriately set a movement amount of the second lens unit caused by the zooming, refractive powers of the second and fourth lens units, and the like. If such configurations are not appropriately set, in a miniaturized zoom lens, it is difficult to obtain high optical performance across the entire zoom range and to maintain a wide viewing angle and a high zoom ratio.
U.S. Pat. No. 7,760,441 and U.S. Pat. No. 7,830,613 respectively discuss retractable zoom lenses having a reduced number of constituent lenses and capable of maintaining a high zoom ratio. In this type of zoom lens, when it is retracted, the overall size thereof can be substantially reduced. However, since a movement amount of the first lens unit is greater than a movement amount of the third lens unit during zooming, a total optical length tends to remain excessively large.
In addition, since the movement amount of the first lens unit during zooming is large, it is necessary to provide a multi-stage lens barrel in order to obtain a thin shape when it is retracted. Therefore, the configuration of the lens barrel becomes complicated, and a size thereof tends to increase in a radial direction. If the refractive powers of each lens surface increase to obtain a thin shape, axial chromatic aberration, chromatic aberration of magnification, and coma increase at the wide-angle end so that correction of such various types of aberration tends to be difficult.
U.S. Pat. No. 7,430,079 discusses a zoom lens having a high zoom ratio of approximately ×10 while it can be miniaturized at the time of retraction. However, the refractive power of the first lens unit increases in order to implement a high zoom ratio, and axial chromatic aberration or chromatic aberration of magnification increase at the wide-angle end so that correction of such various types of aberration tends to be difficult.